The braking system of a “two-mode” hybrid motor vehicle, which vehicle synergistically includes an internal combustion engine and an electrical propulsion system, is accomplished, typically, through two different braking modes: powertrain regenerative braking and standard friction braking. Powertrain regenerative braking is a technology which provides vehicular slowing by converting kinetic energy of the vehicle's movement into electromagnetic energy which is returned to a battery of the electrical propulsion system. Standard friction braking is a technology which provides vehicular slowing by converting the vehicle's kinetic energy into heat by means of friction.
In each of these braking modes, the driver applies force to the brake pedal which is transmitted from the brake pedal pad through a pedal lever arm to a pushrod which delivers the driver applied force as an input to a primary piston of the master cylinder. The motion of the primary piston by the driver's applied force is opposed biasably by a return spring, which is preselected with an initial spring compression load and a particular spring constant which collectively ensure that the primary piston returns to its initial seated position after every brake application by the driver (while seals may provide a small resistance, the initial spring compression load of the return spring is the main determiner of the pedal preload needed for the driver to initiate brake pedal travel).
The vehicular deceleration is a function of brake pedal travel and brake pedal force as determined by software calibration. The brake pedal force feedback to the driver over and above the return spring initial spring compression load is created by the master cylinder pressurizing a pedal “simulator” which consists of a piston and cylinder arrangement, where the piston compresses a spring. The master cylinder starts developing fluid pressure when a lip seal covers the by-pass ports in the primary piston. The onset of braking can be chosen through the calibration to occur just as the fluid pressure starts to build in the master cylinder. The simulator allows for a desired linear increase of force and travel as the vehicle increases its deceleration. The end result is a brake pedal force versus brake pedal travel curve, as for example shown at FIG. 5, plot 304 of graph 300 that starts at the pedal preload target “E”, then linearly increases to a “knee” point “F” just prior to the onset target, then continues to “G” through the target for deceleration at a value of 0.5 g above onset.
In general, (i.e., hybrid and non-hybrid vehicles) the master cylinder converts the physically applied brake pedal force by the driver into hydraulic pressure which operates other devices of the braking system. In a basic hydraulic braking system, the master cylinder creates a pressure output to the friction brakes after a small amount of travel. This travel is between the lip seal and the compensating fluid port to the fluid reservoir. Due to a need for power assist on most passenger vehicles, a brake pedal gain device is utilized, typically either in the form of a vacuum servo booster or a hydroboost unit located between the pedal and the master cylinder. This provides an opportunity to tune the initial force and travel characteristics of the pedal by modifying the internal components of the vacuum booster or the hydroboost.
The foregoing is exemplified by FIGS. 1A through 1C, which depict a prior art master cylinder used in a hybrid vehicle, and which is merely by way of exemplification a GMT900H master cylinder of General Motors Corporation of Detroit, Mich.
The master cylinder 10 receives a force input by the driver from the brake pedal 6 via a pedal lever arm 8 (see FIG. 1C), wherein the force is transmitted to a push rod 12 of the master cylinder. A terminal end 12a of the push rod 12 inserts into a cavity 14a formed in a primary piston 14 and axially abuts the primary piston, typically as a ball and socket arrangement. A terminal end portion 14b of the primary piston 14 passes through a fluid seal 16 and abuts an intermediate piston 18. The intermediate piston 18 is biased toward its initial seated position (the primary and the intermediate pistons being shown at their respective initial seated positions in FIGS. 1A and 1B) by a return spring 20, wherein the biasing is in opposition to brake pedal force supplied by the driver via the push rod 12. The return spring 20 has a predetermined initial spring compression load and a predetermined spring constant so that the primary piston returns to its initial seated position after every brake pedal application by the driver. The master cylinder further includes secondary pistons 22.
In the case of a two-mode hybrid vehicle, the normal operation of the master cylinder generates the working pressure to the friction brakes, which is produced by a hydraulic pump/accumulator setup in response to electrical signals from the brake pedal force and brake pedal travel, so that no brake pedal gain device is needed between the pushrod and the master cylinder. In the case of other hybrids, i.e., larger hybrid SUVs, the pedal feel is modified by the action of a zero-adjust brake switch located at the pedal arm to the pushrod interface which is there to provide a brake switch input. This switch input provides an electrical signal to the secondary braking circuit. Non-hybrid trucks, for exemplar comparison, also use this zero-adjust brake switch for the switch function, and any modification of the pedal feel due to the switch is secondary as the switch is only tuned for switch function, and not specifically tuned for pedal feel.
Historically, engineering of the human interface with a braking system has been a subjective endeavor. With the advent of a Brake Feel Index, hereinafter referred to as “BFI”, as reported in SAE technical paper 940331 “Objective Characterization of Vehicle Brake Feel” by D. G. Ebert and R. A. Kaatz of General Motors Corporation, dated Feb. 28-Mar. 3, 1994, which paper is hereby incorporated herein by reference, a method was developed to correlate objective engineering parameters to these subjective assessments of brake pedal feel as perceived by the driver. In the case of BFI, such aspects as brake pedal application force, brake pedal travel and brake pedal preload are compared to desired target values which correlate to a particular type of response desired and the deviation from these target values is reflected in a lower index value. In the conventional master cylinder, including that shown at FIGS. 1A and 1B, changing these parameters requires extensive reconstruction and retooling thereof.
An existing engineering issue relating to the master cylinder is the return spring initial spring compression load and the primary piston travel before fluid pressure generation, also known as “cut-off” travel, which produce a brake pedal preload and force versus travel relationship that has to be adjusted so as to avoid an undesirable brake pedal feel. The return spring compression load required to ensure that the primary piston will return to its initial seated position per pressure buildup in the master cylinder results in too little force increase during primary piston travel before the “knee” point of the pedal travel versus pedal force curve (see “F” in plot 304 of graph 300 of FIG. 5). The BFI targets are not easily met within hardware constraints, since to meet these targets would require expensive reconstruction and testing of the master cylinder, return spring, and other related hardware of the braking system.
Accordingly, what remains needed in the art is some way to adjust parameters which effect BFI without requiring a complete and comprehensive reconfiguration of the master cylinder, for multiple applications and brake pedal feel requirements of different motor vehicle platforms.